The present invention relates to an automatic focusing system operating with a TV (television) camera for inspecting a surface of an object to be viewed, and, more particularly, to an automatic focusing system operating with a TV camera for insepcting printed patterns on a printed wiring board, making compensation for differences in distances between the TV camera and a part of the printed patterns on the surface of the printed wiring board.
Recently, patterns printed on a printed wiring board are very fine, so that when the printed patterns are inspected using a TV camera, the whole printed wiring board cannot be inspected at the same time. Therefore, a plurality of inspecting points are previously designated on the printed wiring board so that a part, to be inspected, pattern exists around each inspecting point. The printed patterns on the printed wiring board are inspected by sequentially aligning a line of sight of the TV camera with the inspecting point, by point. Furthermore, recently, a printed wiring board, which will be called simply "board" hereinafter, has scaled up and a thickness of the board increases to include multiple layers. Accordingly, phenomena such as warp and locally varied thickness occur in the board. Such phenomena produce a difference among distances from the TV camera to respective inspecting points. (The distance mentioned above will be called "inspecting camera distance" hereinafter.) For example, when inspecting the board having a size of of 300 mm.times.300 mm and large layers such as 14 layers, the inspecting camera distance varies as much as1.5 mm at maximum. When, thus, the size of the board is large, many inspecting points are required to be designated on the board, and focusing of the TV camera for each inspecting point must be performed "automatically" at a high speed.
Recently, in the automatic focusing system, an active means such as an infrared ray or ultrasonic wave is used for measuring the inspecting camera distance. That is, in the prior art, the TV camera is automatically focused by measuring the inspecting camera distance by using the infrared ray or an ultrasonic transmitting and receiving apparatus with the TV camera. Furthermore, in the prior art, inspecting a printed pattern around an inspecting point and shifting the board so as to align the line of sight of the TV camera with a next inspecting point are also performed automatically. Such automatic inspection of the printed patterns on the board has been heretofore performed in the prior art in accordance with the following steps: (1) aligning the the TV camera with an inspecting point by shifting the board, for example, in X and Y directions; (2) measuring the inspecting camera distance by the infrared or ultrasonic transmitting and receiving apparatus; (3) adjusting a focusing mechanism of the TV camera, using the measured distance (4) inspecting a printed pattern around the inspecting point; (5) aligning the the TV camera with a next inspecting point by shifting the board; and (6) repeating the above steps from (2) to (5) until the printed patterns around the entire number of inspecting points are inspected, in other words, until the pattern inspection of the board is over.
When the printed patterns on the board are very fine, many inspecting points are necessary for inspecting the printed patterns, and a very long period of time is required for completing the inspection of the printed patterns, resulting in an increased fabrication cost of the board. Generally, the number of inspecting points depends on a size of the board, a density of printed patterns on the board and a characteristic of the TV camera, such as a field of view (FOV) and a resolution thereof. For example, when the board has a size of 300 mm.times.300 mm, the printed patterns have a density such that wiring patterns, each having a width of 0.2 mm, are arranged with 0.5 mm pitch. Typically when a SONY XC-38 type TV camera is used at least 2,000 inspecting points are required on one side surface of the board (4,000 inspecting points are required on both side surfaces of the board). Wherein, the SONY XC-38 type TV camera has an FOV such that an area of 2.56 mm.times.2.56 mm in size can be viewed on the surface of the board and a resolution that can be resolved into 491.times.384 picture elements. In the above case, to perform the steps (1) or (5), and (2) and (3), a time of approximately 3 sec is required (approximately 1 sec for each); incidently, approximately 3 sec are required for the real inspection in the step (4), so that a time of 6 sec is required for inspecting the printed pattern around one inspecting point. Therefore, a time of 6,000 sec (3 sec/inspecting point .times.2,000 inspecting point=6,000 sec=1 hour and 40 minutes), is required to the steps (1) or (5), and (2) and (3), and a time of 12,000 sec (6 sec/inspecting point .times.2,000 inspecting point=12,000 sec=3 hours and 20 minutes) is required for inspecting the printed patterns on one side of the board. Taking such a long time for inspecting the printed patterns on one side surface of the board reduces; the productivity and increases production costs of the boards. Therefore, how to reduce this long time has been a big problem in the prior art. If the time required to perform the step (2) (for measuring the distance) is saved to zero, 33 minutes can be reduced. The present invention intends to reduce the time for the step (2) to zero.